1. Field of the Invention
The present invention relates to a torque sensor for detecting a torque transmitted to a rotating shaft without direct contact with the rotating shaft.
2. Description of the Prior Art
There has been an increasing demand for non-contact detection of a torque which is transmitted to a rotating shaft.
One example of conventional non-contact detecting type torque sensors is disclosed in Japanese Patent Laid-open Publication No. 59-77326. The disclosed torque sensor includes a magnetostrictive amorphous magnetic alloy film formed by bonding or plating on an outer peripheral surface of a shaft. The amorphous magnetic alloy film has two juxtaposed portions formed symmetrically in helical forms slanting at +45.degree. and -45.degree., respectively, with respect to a longitudinal axis of the shaft. A pair of coils are wound around the respective helical portions of the amorphous magnetic alloy film, with a predetermined space between the coils and the amorphous magnetic alloy film, Thus, a magnetic circuit is formed by the coils and the amorphous magnetic alloy film.
With this arrangement, when a torque is transmitted to the shaft, the outer peripheral surface of the shaft is strained or otherwise deformed. In this instance, if the torque acting on the shaft is clockwise, one helical portion of the amorphous magnetic alloy film which is slanting at an angle of +45.degree. increases in its magnetic permeability, while the magnetic permeability of the other helical portion slanting at an angle of -45.degree. decreases. This relation in magnetic permeability is reversed when the torque acts on the shaft in the counterclockwise direction. The difference in self-inductance between the two coils is measured by a differential detector whereby the direction and magnitude of the torque can be detected.
Another known non-contact detection type torque sensor is disclosed in Japanese Patent No. 169326, which uses a shaft having magnetostrictive properties. The shaft has two helical grooves formed symmetrically in its outer peripheral surface. One of the helical grooves has a positive pitch relative to the longitudinal direction of the shaft, while the other helical groove has a negative pitch relative to the longitudinal direction of the shaft. Two coils are wound around the respective grooved shaft surface portions in concentric relation to the shaft. When a torque is transmitted to the shaft, the shaft undergoes magnetostriction. The magnetic permeability of the grooved shaft surface portions changes in opposite directions, thereby changing self-inductance values of the respective coils which are wound around the grooved shaft surface portions. The difference in self-inductance between the two coils is electrically detected for simultaneously determining the direction and magnitude of the torque in a non-contact manner.
In the first-mentioned known torque sensor, the magnetostrictive amorphous magnetic alloy film is formed directly onto the outer peripheral surface of the shaft. With this construction, when the shaft surface is greatly strained by a torque, magnetic characteristics of the torque sensor are saturated and hence the torque sensor becomes insensitive to the torque. In addition, the amorphous magnetic alloy is likely to peel off or crack and, hence, the durability of the torque sensor is relatively low. The foregoing drawbacks may be overcome by enlarging the diameter of the shaft in such a manner as to reduce the amount of strain of the shaft surface against the same torque. Such an attempt is, however, impracticable because the shaft is excessively heavy. A magnetic layer may be formed on the shaft surface by bonding, plating or sputtering. This attempt is not satisfactory due to the necessity of an apparatus which is large in size and difficult to operate.
The last-mentioned known torque sensor depends on magnetostriction of the shaft and hence must grantee a certain degree of durability of the shaft when used for detection of a large torque. In order to provide the necessary durability, the magnetostrictive shaft is strengthened either by hardening or by enlarging its diameter, thereby lowering stresses created on the shaft surface. The hardening is effective to increase the mechanical strength of the shaft but causes a problem of deterioration of magnetic properties of the shaft. This process is, therefore, unable to realize a high precision torque sensor. On the other hand, the enlargement of the shaft diameter results in an excessively heavy shaft which is not suitable for practical usage and is difficult to handle when the helical grooves are formed in the shaft.
In addition, both of the above-mentioned know torque sensors have a problem that an apparent torque is developed when the shaft is rigidly assembled with an apparatus to which the torque sensor is applied.